The novolac-based resins used as positive-tone photoresists are frequently etched in an oxygen plasma. II is desirable to have a predictive model of the photoresist etch rate But. for process improvement, control, and analysis, the development of a rigorous mechanistic model is impractical. Instead, a simplified mechanistic model is derived, here, according to the method proposed by Hougen and Watson for the study of fluid-solid interactions This model derivation method is employed in order to arrive at a functional form that represents chemical etching of the resist by oxygen radicals, assisted by the plasma ion flux. Values for model parameters are determined from process data By nonlinear regression. The quality of the model fit M the data is rested statistically.